promissory adj.

c. 1400, promisse, "a solemn pledge; a vow; a declaration in reference to the future made by one person to another, assuring the latter that the former will do, or not do, a specified act," from Old French promesse "promise, guarantee, assurance" (13c.) and directly from Latin promissum "a promise," noun use of neuter past participle of promittere "send forth; let go; foretell; assure beforehand, promise," from pro "before" (from PIE root *per- (1) "forward," hence "in front of, before") + mittere "to release, let go; send, throw" (see mission).

Sense of "that which affords a basis for hope or expectation of future excellence or distinction" is by 1530s.